Demand for smaller and more sophisticated electronic devices continues to drive the electronics industry towards improved integrated circuit package types. Improved packages typically must be capable of supporting higher lead counts while at the same time minimizing package volume.
The ball grid array, or BGA, is one improved package type receiving ever increasing industry acceptance. In a ball grid array, a die attach adhesive attaches the lower surface of a semiconductor die to the upper surface of a printed wiring board-like laminate substrate. Wire bonds complete electrical connections from the upper side of the die to solder contact points on the upper substrate surface. The solder contact points on the upper substrate surface are connected to a typically square array of solder contact points on the lower substrate surface through a plurality of conductive circuit elements and through holes, or "vias" which penetrate the substrate. A plurality of metallic balls are soldered to the lower substrate surface to form an array of conductive ball elements suitable for surface mount attachment of the BGA to a mother board.
The relatively fragile wire bond connections running from the top of the semiconductor die to the upper surface of the BGA substrate must be encapsulated to maintain the integrity of the connections. While it is possible to encapsulate the die by applying liquid materials over the die and laminate to form a "glob top" over the die and laminate, such glob top encapsulation techniques generally are slow and aesthetically unfavored. BGA manufacturers therefore prefer to encapsulate BGAs with low warpage thermosetting mold compounds.
BGA mold compounds differ from encapsulants used for other, more conventional molded semiconductor packages. The BGA's thin, wide geometry and its high lead count demand a mold compound exhibiting low warpage and excellent moldability, as well as low viscosity, high spiral flow, little or no flash and bleed, and a low coefficient of thermal expansion below the mold compound's glass transition temperature. BGA mold compounds also differ from those used for more traditional packages in that in addition to adhering to the semiconductor die and package leads, the BGA mold compound also must adhere strongly to the upper surface of the BGA substrate.
The BGA substrate upper laminate surface typically is a polymeric solder resist material. Mold compound adhesion to this surface is affected by the texture and cleanliness of the surface and by the physical properties of the mold compound. To ensure that the mold compound adheres to the laminate surface, manufacturers typically perform a cleaning step to prepare the substrate surface to receive the mold compound.
Plasma cleaning is the predominant method for preparing BGA substrate surfaces prior to overmolding of the package with mold compound. Plasma cleaning of BGA substrate typically requires placing the substrate in a vacuum chamber, and then passing a gas containing ionized or excited gas molecules over the substrate to clean the surfaces that will receive molding compound. The production cost of BGA devices could be reduced substantially if manufacturers could employ mold compounds that would strongly adhere to BGA substrates without plasma cleaning.
Current state of the art BGA molding compounds typically employ multifunctional epoxy resins, multifunctional hardeners, flame retardants, silica fillers, silicone rubber and silicon fluids. Each of these components is believed to be necessary in combination to provide the moldability, warpage, shrinkage, and low stress characteristics required by BGA manufacturers.
Multifunctional resins and hardeners are needed to impart high temperature flexural strength to the mold compound and to reduce solder-induced cracking.
Silica fillers are required to reduce moisture absorption by the mold compound and to lower the mold compound's coefficient of thermal expansion.
Silicone rubber particles reduce the coefficient of thermal expansion and the modulus of the mold compound, while silicone fluids are believed to be necessary to reduce the modulus of the mold compound and to decrease the viscosity of the molding compound.
Unfortunately, BGA mold compounds as described above suffer a marked inability to adhere to polymer-coated BGA substrate in the absence of a cleaning step such as plasma cleaning.
Additionally, BGA mold compounds as described above also have been known to exhibit relatively low break strength when formed into the small, typically cylindrical shapes known as "preforms." Such preforms are the preferred form for handling mold compound used in semiconductor transfer molding applications, and preform breakage can lead to dust, which in turn can interfere with molding and reliability of the semiconductor devices.
What is needed, therefore, is a mold compound which exhibits high preform strength, excellent moldability, minimal warpage, and the low shrinkage and low stress characteristics required by semiconductor manufacturers, while at the same time eliminating a need for premolding cleaning of BGA components.